1. Field of the Invention
The invention relates to amorphous metal alloy compositions, and, in particular, to compositions including substantial amounts of one or more of the elements of Mo, W, Ta and Nb, which evidence both high crystallization temperatures and high hardness values.
2. Description of the Prior Art
Investigations have demonstrated that it is possible to obtain solid amorphous metals for certain alloy compositions, and as used herein, the term "amorphous" contemplates "solid amorphous". An amorphous substance generally characterizes a noncrystalline or glass substance; that is, a substance substantially lacking any long range order. In distinguishing an amorphous substance from a crystalline substance, X-ray diffraction measurements are generally suitably employed. Additionally, transmission electron micrography and electron diffraction can be used to distinguish between the amorphous and the crystalline state.
An amorphous metal produces an X-ray diffraction profile in which intensity varies slowly with diffraction angle. Such a profile is qualitatively similar to the diffraction profile of a liquid or ordinary window glass. On the other hand, a crystalline metal produces a diffraction profile in which intensity varies rapidly with diffraction angle.
These amorphous metals exist in a metastable state. Upon heating to a sufficiently high temperature, they crystallize with evolution of a heat of crystallization, and the diffraction profile changes from one having glassy or amorphous characteristics to one having crystalline characteristics.
It is possible to produce a metal which is a two-phase mixture of the amorphous and the crystalline state; the relative proportions can vary from totally crystalline to totally amorphous. An amorphous metal, as employed herein, refers to a metal which is primarily amorphous; that is, at least 50% amorphous, but which may have a small fraction of the material present as included crystallites.
For a suitable composition, proper processing will produce a metal in the amorphous state. One typical procedure is to cause the molten alloy to be spread thinly in contact with a solid metal substrate, such as copper or aluminum, so that the molten metal rapidly loses its heat to the substrate.
When the alloy is spread to a thickness of about 0.002 inch, cooling rates of the order of 10.sup.6 .degree. C/sec may be achieved. See, for example, R. C. Ruhl, Vol. 1, Mat. Sci. & Eng., pp. 313-319 (1967), which discusses the dependence of cooling rates upon the conditions of processing the molten metal. For an alloy of proper composition and for a sufficiently high cooling rate, such a process produces an amorphous metal. Any process which provides a suitably high cooling rate can be used. Illustrative examples of procedures which can be used to make the amorphous metals include rotating double rolls, as described by H. S. Chen and C. E. Miller, Vol. 41, Rev. Sci. Instrum., pp. 1237-1238 (1970), and rotating cylinder techniques, as described by R. Pond, Jr. and R. Maddin, Vol. 245, Trans. Met. Soc., AIME, pp. 2475-2476 (1969).
Amorphous alloys containing substantial amounts of one or more of the elements of Fe, Ni, Co, V and Cr have been described by H. S. Chen and D. E. Polk in a patent application, Ser. No. 318,146, filed Dec. 26, 1972, now U.S. Pat. No. 3,856,513, issued Dec. 24, 1974. Such alloys are quite useful for a variety of applications. Such alloys, however, are characterized by a crystallization temperature of about 425.degree. C to 550.degree. C and a hardness of about 600 to 750 DPH (diamond pyramid hardness).